Class-A or highly biased class-AB operation of power amplifiers is desirable for audio signal amplification because it eliminates or reduces crossover distortion and improves the linearity of a power output stage. In a class-A amplifier, the bias and alternating input signal applied to the input electrode (gate or base) of a transistor amplifying element is such that current flows through the output electrodes (source and drain, or emitter and collector) at all times. In Class-AB operation, the bias is such that current flows for more than half but less than the entire input cycle. In Class-B operation, output current flows only during the positive (or negative) half cycle and does not flow through the transistor during the opposite half cycle, so that the transistor regularly switches on and off, causing distortion. However, while the fidelity is improved with Class-A or Class-AB operation, it also has a disadvantage that it is very inefficient, resulting from its normally high idle current. For example, a conventional push-pull complementary output stage operated in Class-A mode will typically idle at more than twice its maximum continuous output rating.
Several techniques have previously been used to reduce the power dissipation in highly biased output stages. In U.S. Pat. No. 3,995,228, an amplifier with a complementary-symmetry emitter-follower output stage is provided with an active bias circuit in place of a constant voltage generator in order to continuously adjust the bias voltage in response to the voltage level at the output mode to maintain forward bias on both output transistors at all times under normal signal conditions. The active bias circuit produces as little increase in the required forward bias as possible for a given current increase, and allows the bias across the "unused" output transistor in the complementary pair to decrease, although not to the point where the transistor would shut off, thus saving power. The active bias circuit includes a pair of V.sub.BE multipliers with a shared voltage divider resistor therebetween, and also includes a pair of constant voltage sources (diodes) connected between the output node of the amplifier and voltage reference nodes associated with each V.sub.BE multiplier.
In U.S. Pat. No. 4,115,739, Sano et al. describe a power amplifier having two amplifier stages and two separate power supplies. A first amplifier stage has an operating point for Class-A operation and drives a load in response to an input signal. A first power supply provides a voltage to the first amplifier stage. The reference point or voltage of the first power supply is floating and is driven by a second amplifier stage. The second amplifier stage has an operating point for Class-B operation and provides the voltage reference for the first power supply at its output node. A second power supply with a fixed reference point (ground) supplies a voltage to the second amplifier stage. Thus, the second power supply is designed to support the full voltage swing of the amplifier, while the smaller first power supply provides only enough voltage to bias the first (output) amplifier stage. The amplifier stages are interconnected so that the Class-B second amplifier stage, fed by the high voltage second power supply, drives the midpoint of the smaller first power supply, which in turn feeds the Class-A output stage. The result is that the high bias of the Class-A output stage does not flow through the high voltage power supply and efficiency is greatly improved. In U.S. Pat. No. 4,206,419, Yokoyama applies a similar technique to an amplifier operated in a common emitter mode with negative feedback. The Sano et al. patent uses emitter-follower (common collector) amplifier stages without feedback. Both employ complementary-symmetry push-pull amplifier stages.
Cascode operation is a well known technique for reducing distortion in linear gain stages. Cascode amplifiers use two transistor amplifying devices, one a common collector (drain) or common emitter (source) gain device and the other a common base (gate) cascode device. The connection between them is such that the collector (drain) of the first device drives the emitter (source) of the common base (gate) device, which provides a low impedance load and holds the voltage across the first device relative constant. The transistor devices can be either bipolar (with bases, emitters and collectors) or field effect transistors (with gates, sources and drains).
An object of the present invention is to provide a high efficiency cascode transistor power amplifier for high fidelity audio signal amplification with reduced power dissipation.